The present invention relates to an absorbent chemithermomechanical pulp and to a method of manufacturing the same.
Hitherto, it has only been possible to apply the process of defibering chips with a low energy input subsequent to preheating the chips under high pressure and high temperature (150-170xc2x0 C.), the so-called Asplund process, within the board manufacturing industry, since the pulp resulting from this process is dark in colour and cannot be bleached at reasonable chemical consumptions. Furthermore, the fibres become coated with a lignin skin and are therefore stiff and rigid, which results in poorer strength and absorption properties. Consequently, it has only been possible to produce chemithermomechanical pulp (CTMP) of high brightness and good absorbency by preheating and refining at a temperature of at most 140xc2x0 C. High brightness is especially important when producing tissue pulp.
DE-A-27 14 730 describes a process for producing a chemically modified thermomechanical pulp where the wood material is preheated at a temperature of 135-200xc2x0 C. during 1-30 minutes. The time used according to the examples is of the order of 10 minutes. To obtain the desired flexibility an energy input of twice the normal is required.
The object of the present invention is to provide a chemithermomechanical pulp which exhibits a low resin content, an extremely high long-fibre content, an extremely low short-fibre content, and an extremely low shive content. Such pulps are particularly suited for the manufacture of fluff and tissue. The extremely low shives content is of special importance when producing tissue pulp. The extremely high long-fiber content with the corresponding high freenes is of special importance when producing fluff pulp. A further object of the invention is to provide a novel method for the manufacture of absorbent chemithermomechanical pulps at low energy inputs.
The invention thus relates to an absorbent chemithermomechanical pulp produced from lignocellulosic material at a wood yield above 88%, a resin content beneath 0.15%, calculated on the amount of resin which can be extracted in dichloromethane, a high long-fibre content, a low short-fibre content and a low shives content, the pulp being characterized in that when fractionating the pulp according to Bauer McNett, the long-fibre content is above 70%, preferably above 75% of fibres retained on a wire gauze of size 28 mesh and the short-fibre content is beneath 10%, preferably beneath 8%, of fibres which pass through a wire gauze of size 200 mesh according to Bauer McNett; and in that the shive content is lower than 3%, preferably lower than 2%, measured according to Sommerville.
The pulp should have such brightness that it can be bleached at a reasonable consumption of bleaching chemicals to a brightness of at least 65% ISO, preferably 70%. Alternatively the pulp may have been beached to such brightness.
This pulp is particularly well suited for the manufacture of fluff and tissue. W
hen the pulp is a fluff pulp it is preferably refined to a freeness of 740 ml at the lowest especially 750 ml at the lowest and suitably 760 ml CSF at the lowest. Such a pulp does not need to be bleached and may have a brightness of at least 45% ISO.
When the pulp is a tissue pulp it has suitably a brightness of at least 65% ISO, preferably above 70%. The tissue pulp does not need to have as high a freenes less than as the fluff pulp. Suitably it is refined to a freeness of 650 ml CSF at the lowest.
The problem with manufacturing pulp suitable for fluff and tissue by means of a chemithermomechanical method lies in the desired combination of high freeness, high long-fibre content, low shive content and high brightness. An increase in temperature when preheating will favour the reduction in shive content but, at the same time, impair brightness.
It has now surprisingly been found that a chemithermomechanical pulp having the desired properties can be produced by
a) impregnating the chips with sodium sulphite, sodium dithionate, alkaline peroxide or the like, with an addition of a complex builder;
b) preheating the chips;
c) defibering the chips to pulp in a refiner at substantially the same pressure and temperature as those employed in the preheating process; and
d) washing and dewatering the pulp to, e.g., a consistency of 25-50%, wherein, in accordance with the invention, impregnation and preheating of the chips is effected in one and the same vessel over a combined treatment time of at most 2 minutes, particularly at most 1 minute, preferably at most 0.5 minute; and
a) using a warm impregnating liquid having a temperature of at least 100xc2x0 C., suitably at least 130xc2x0 C. and preferably having essentially the same temperature as that of the preheating process;
b) preheating the chips at a temperature of 150-175xc2x0 C., preferably 160-170xc2x0 C.; and
c) carrying out the defibering process with an energy input which is at most half of the energy input required for defibering to the same shive content in a similar refiner when preheating and defibering are performed at 135xc2x0 C.
The complex builder used in the impregnating process may, for instance, be DTPA, which contributes to an improvement in pulp brightness.
The pulp may e) be refined to a brightness above 65% ISO, preferably above 70%. To accomplish this at a reasonable consumption of bleaching chemicals the brightness after refining has to be at least 45% ISO, preferably at least 50%. Such bleaching should preferably be performed when the pulp is a tissue pulp.
In order to obtain a pulp of sufficient brightness, it is essential that preheating at the aforesaid high temperature is not permitted to proceed over a period of time of as long a duration as the standard preheating time of about 3 minutes used when producing chemimechanical pulp of CTMP type. In order to enable the preheating time to be lowered to at most 2 minutes, preferably at most 1 minute, it is necessary to use an impregnating solution which is heated to a temperature of at least 100xc2x0 C., particularly at least 130xc2x0 C. and preferably substantially to the same temperature as that used in the preheater. Furthermore, no impregnating liquid shall be removed between the impregnating and preheating steps. Consequently, impregnation is effected in the same vessel as that in which the chips are preheated, and at the same pressure and suitably at the same temperature or only a slightly lower temperature. The brightness of the pulp is sustained because of the very short stay time at the high temperature, so that an excessively large quantity of bleaching chemicals, such as peroxide, will not be required in the following bleaching step. Furthermore, the wood yield obtained in this way is almost equal to the wood yield obtained when preheating the chips conventionally at 130-140xc2x0 C. In addition, when refining to a freeness slightly above 750 ml CSF, the energy input required for the defibering process is reduced from about 600 kWh/tonne at 130xc2x0 C. to less than 300 kWh/tonne at. 170xc2x0 C. These values,have been obtained in a pilot plant. Commercial values may differ from those obtained at pilot level. The relative differences between the levels for shives content, brightness and energy input obtained in the pilot plant at conventional temperature and at the temperature according to the invention, respectively, should, however, remain in a commercial plant.
The inventive method suitably includes the conventional steaming, impregnating, preheating, defibering, washing, screening, washing, possibly bleaching, washing and drying stages. Whereas a conventional impregnating process is carried out with cold liquid in a vessel other than the preheating process, which is carried out over a period of about 3 minutes and at a temperature of about 130xc2x0 C., and in which process impregnating liquid is removed between the impregnating stage and the preheating stage, the impregnating and preheating processes of the inventive method are combined in one and the same vessel and are carried out at the same pressure and substantially the same temperature 100-175xc2x0 C., 150-175xc2x0 C. respectively, over a combined time period of at most 2 minutes, suitably at most 1 minute and preferably at most 0.5 minute.
Because preheating is effected at high temperature, the refining process requires less energy. A low energy input will normally result in high freeness and high shive content. A surprising characteristic of the present invention is that at low energy inputs, success is achieved in combining high freeness with low shive content. Low energy input would otherwise result in a high shive content.
When applying the inventive method in tests on a laboratory seal, a freeness of above 780 ml CSF was achieved with an acceptable shive content. In some instances, a freeness of above 800 ml was achieved. This can be compared with a freeness of about 650-750 ml CSF in the normal production of CTMP-fluff.
The pulp is washed subsequent to the refining process, suitably under pressure and at high temperature, preferably while excluding air from the system and in immediate connection with the refining stage. The pulp is dewatered to a consistency of e.g. 25-50%. Possible bleaching is then carried out with peroxide or other bleaching chemical. If desired, the pulp can again be washed, after the bleaching process.
When producing fluff, defibering is carried out to a freeness of 740 ml at the lowest, suitably of 750 at the lowest, preferably of 780 ml CSF at the lowest. When producing tissue pulp the refining may be carried out to a freeness of 650 ml CSF at the lowest.
When applying the inventive method, it is possible to produce pulp with a wood yield above 88%, preferably above 90%, a resin content of less than 0.15%, calculated on the. amount. of resin that can be extracted in dichloromethane, and a brightness above 65% ISO after bleaching.